Transcript Channel Access Concept

EPICS
Channel Access Concepts
Kazuro Furukawa, KEK, (2000-2004)
<kazuro.furukawa @ kek.jp>
(Bob Dalesio, LANL, USPAS1999)
(Andrew Johnson, APS, USPAS2003)
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Why Channel Access?

We don’t have
Network transparent computing in general (yet)


Plenty of administration tasks for sharing data
No standard way for remote processing


EPICS
RPC, COM, CORBA, RMI, .NET, etc.
In EPICS, Channel Access provides
Efficient Software Bus on Network
Performance, Rapid development, Maintenance, Administration
Tuned for each operating system
(But no internal documentation yet)
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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EPICS Provides Interfaces at All Levels
EPICS
Archive Viewing Archive Analysis
Program Interfaces
User Tools
DM/MEDM/DM2K/EDM/JDM
Alarm Handler / stripTool/
knobManager
Connection Server
Archive Access
Archiving
Channel Access Client (CAC)
Connection Data Transfers
C, C++, Fortran, JAVA
IDL / Matlab / Mathematica
ActiveX / DDE / VisBasic
SDDS / SAD /
tcl / PERL / Python
State Notation Lang / FSQT
WAN/LAN/Local
Connection
Data Transfers
Channel Access Server (CAS)
DB Engine Types
Connection
Data Transfers
Portable Channel Access Server (CAS)
Generic Data Descriptor
Access Lib
Active X
Channel Access Client
Gateway
Any Data
Store
Portable CAS
Labview / Excel..
PCAS - Windows
Device Support
Driver Support
I/O Controller
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Channel Access :
The EPICS Software Bus
EPICS
Operator Interface
Archive Tool
Alarm Tool
Application
Application
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Control Processing
Functionalities




On-demand Synchronous
On-demand Asynchronous
Periodical
On-Change






EPICS
(with value deadband)
(with rate limit)
On-Event (Trigger)
Cache
List of Control Points
Static Information and History using the Same API
Existent Control Systems Support a Part of Those
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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What is Channel Access (CA)
EPICS

Standardized communication path to a named data in
a Channel Access server, usually a field within a
record (process variable) in any IOC database.

This data is a communication path to/from a
hardware IO channel or other software.

Integrates software modules into the control system.

A callable interface (library of subroutines).
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Why Use Channel Access
EPICS

Callable interface designed for easy use by casual applications

Callable interface also designed for use by system software
components such as the operator interface, sequencer, and the
archiver

Operating system transparency

Network transparency
(access to remote and local channels is identical)

CPU architecture independence (silent data conversion)

Isolation from system software changes.

Efficiency (host IO channels)

Efficiency (network remote IO channels)
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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EPICS
Channel Access network
architecture
Operator
Interface 1
...
CAC
Operator
Interface n
CAC
Data logger,
Data Analysis
CAC
CAC
CAC
IEEE 802.3
CAS
CAC
CAC
CAS
CAS
...
IO Controller 1
IO Controller 2
IO Controller n
CAS Channel Access Server
CAC Channel Access Client
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Client Server Model
EPICS

CA is a network service.

Clients use the callable interface (link to the CA
library).

Server replicated in each IOC (part of iocCore).

Clients make requests to the servers across the
network.

CA defines a network protocol.

Local operations are initiated and performed by the
host processor.

Remote operations are initiated but not performed on
the host processor.
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Asynchronous Nature of CA
EPICS

CA does not wait to gain access to the network prior to
returning from each library call.

Remote operation requests are buffered and sent when the
buffer fills or when you ask.

Data fetched from a remote machine is generally not
immediately available.

With few exceptions values written into your variables by
CA should not be referenced until you have synchronized.

All operations guaranteed to be executed in the order
requested.
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Why is CA Asynchronous?
EPICS

Combined operations are more efficient when sharing a
common resource such as a bus.

Combined operations can be more efficient when passing
through system software layers.

Sometimes it is useful to perform labor on a local
processor while several operations are completing on
remote processors.

The plant is often asynchronous.
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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CA Performance
EPICS
2000
microseconds
1800
1600
local access
1400
remote asynchronous access
1200
remote synchronous access
1000
800
600
400
200
0
Reads
Writes
Simple Unconverted I/O Channel Reads and Writes
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Methods of Synchronizing

EPICS
No data fetches are left outstanding after completing a
call to ca_pend_io().
 Asynchronous
replacement for ca_get():
ca_get()
ca_pend_io()

Use a monitor.

Use fetch with callback.

Use a synch protocol with a remote program.
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Event Propagation
EPICS

In any process control system, an application
program must be prepared to respond to any one of a
number of asynchronous events.

Events include hardware or software state changes
(limit switches, flow indicators, out of range analog
channels, software exceptions, etc.)
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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CA Software Interface to Events
EPICS

An event is a significant change in either the value of
a field or the condition of the record as a whole.

Events are placed in a queue and handled in the
order that they occurred.

A channel can be monitored by specifying a handler
to be run each time an event occurs.

CA client applications using events tend to be tree
structured.

Updating the client’s local value for a channel this
way can save on network traffic since a message
soliciting the update need not be sent.
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Event Rate Management
(Analog Channels)
EPICS

The rate at which updates are sent over the network
should be minimized by the project engineer within
reasonable constraints. This rate is managed by adjusting
the channel’s deadband and scan rate.

(R3.15 may provide facility to limit monitor rate (?))
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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CA Exceptions
EPICS

Since the CA client library does not wait to gain access to
the network prior to returning from each call an operation
can fail in the server after the library call that initiated it
returns.

Status of these unsuccessful operations are returned from
the server to the client’s exception handler.

The default exception handler prints a message for each
unsuccessful operation and aborts the client if the condition
is severe.

Operations which fail in the server are nearly always
caused by programming errors.
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Channel Naming Convention

CA requires that channels have a name.

The IOC database requires names of the form:

EPICS
<record name>[.<field name>]

rfhv01.LOPR”

“rfhv01”

Record names are assigned by project engineer following project
naming convention.

Record field names and purposes are record type specific

A list of the field names available for each record can be obtained from
the database documentation (EPICS Record Reference Manual.)

If the field name is omitted, the field .VAL is assumed. This field contains
a control or read back value.
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Native Data Types
EPICS

All channels have a “native” data storage type in the IOC.

All native data storage types are “atomic”.

Atomic data types include:
 integer,
floating point, string, enumerated etc.

When transferring a new value to/from a channel the
client program specifies the data format to supply/receive
it in. This is often referred to as the external data type.

The external data type can be different from the native
type if conversion is possible.
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Compound Data Types

EPICS
Compound data types contain a channel value combined
with additional status or configuration information.
 Value
+ Alarm Status + Timestamp + Drive Limits

Compound types involve the database record as a whole.

Compound types can currently only be used with gets and
monitors.

Data types are described in db_access.h. (DBR_XXXX)
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Connection Management
EPICS

Network Connections are inherently transient.

A channel’s connection state is either not found,
connected, or disconnected.

CA allows you to specify a handler to be run when a
channel’s connection state changes.

Connection requires a server for the channel and a valid
network path to the server.

CA automatically restores connection upon notification
from the server.
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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Software Releases
EPICS

IOC core and CA client library EPICS major release
number must match, or client will not find server.

This is due to potential CA protocol redesign between
major releases.

CA protocol is upwardly compatible within a major release.

When new features are added to the client, older versions
of the server won’t support them.
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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EPICS
WORKSTATION (OPI)
user program
Channel Access
Repeater
Channel Access
server
Channel Access
client
Channel Access
client
database access library
C program
SNL sequence
DATABASE
database library
record support
device support
device drivers
IOC
instrumentation
and
control hardware
Channel Access Concepts – EPICS Training – K.Furukawa – Mar.2004.
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